Lactic acid tolerance determined by measurement of intracellular pH of single cells of Candida krusei and Saccharomyces cerevisiae isolated from fermented maize dough

Abstract

Strains of Candida krusei and Saccharomyces cerevisiae were grown together at 30jC in MYGP broth, pH 2.5, in the presence of 106.4 mM undissociated lactic acid. The two C. krusei strains investigated grew within 48 h from initial counts of 2 104 to approximately 107 cells/ml whereas the two S. cerevisiae strains investigated survived but did not grow in the presence of 106.4 mM undissociated lactic acid at pH 2.5. To explain the differences in lactic acid tolerance of the two yeast species, we used fluorescence-ratio-imaging microscopy and a perfusion system to determine the short-term intracellular pH (pHi) changes in single cells of C. krusei and S. cerevisiae. The changes were investigated both in the presence of low (20.7mM) and high (106.4 mM) concentrations of undissociated lactic acid. For both the investigated species 20.7 mM undissociated lactic acid did not seem to influence the initial pHi which for C. krusei was found to be approximately 8.0 and for S. cerevisiae 6.9–7.5. For both C. krusei strains, perfusion with 106.4 mM undissociated lactic acid induced only weak short-term pHi responses with a decrease in pHi of less than one pH unit. Contrary, for both strains of S. cerevisiae perfusion with 106.4 mM undissociated lactic acid resulted in a significant decrease in pHi from initially 6.9–7.5 to 6.2–6.4 after 1 min and further to a pHi of V5.5 after 3 min after which it remained constant. The results obtained show that C. krusei is more resistant to short term pHi changes caused by lactic acid than S. cerevisiae, and this, in turn, may be part of the explanation why C. krusei is more tolerant to lactic acid than S. cerevisiae